Polypeptide hormones engage cell surface signaling receptors to regulate an enormously diverse array of physiological functions. Our body fluid volume and blood pressure, termed volume homeostasis, are regulated, in large part, by a family of secreted natriuretic peptide (NP) hormones and their receptors (NPR), whose signaling is coupled to guanylyl cyclase (GC). In organs such as the heart, kidney, intestine, eye, and vascular system, the interactions of NPs with NPRs serve multiple and coordinate roles in mobilizing water excretion and ion transport. Disregulation of the components of this hypotensive system manifests in pathological conditions such as high blood pressure (hypertension), congestive heart failure, renal failure, and infectious diarrhea. These diverse maladies are linked by an inability to effectively manage fluid volume. In fact, the importance of the NP system as a therapeutic target was validated in 2001 by the introduction of Brain Natriuretic Peptide (BNP) into the clinic (Natrecor(r)), as the first new treatment for congestive heart failure in 15 years. Numerous other indications for NP-targeted therapy exist, most recently in colon cancer and angiogenesis. Our hypothesis is that understanding the molecular basis of NP hormone recognition, and the subsequent intracellular mechanisms of NP receptor activation, are necessary to eventually achieve the most effective modulation of this system. We propose that focused structural, biochemical, and pharmacological studies of the extracellular NP receptor-ligand interactions, in tandem with the intracellular NP receptor signaling modules, will elucidate a complete mechanistic understanding of the allosteric transitions which couple hormone recognition to receptor activation. We will express soluble, recombinant forms of NPR extracellular (ECD) and intracellular (ICD) domains for biochemical characterization of hormone recognition, and guanylyl cyclase activation, respectively. We will determine the three-dimensional structures of the quiescent (unliganded) and active (complexed) ECD and ICD by x-ray crystallography, in order to visualize the receptor-bound conformations of the NP polypeptide hormones, as well as ligand-induced conformational changes in the receptors. We will then utilize these results to design modified, receptor-selective variants of NP hormones with a potential clinical utility. Hence, the ultimate goals of this project are both fundamental and applied. The structural studies will elucidate novel mechanistic paradigms about receptor signaling, which will have direct applications to the therapeutic goal of modulating volume homeostasis in a variety of edematous disorders.